Why does my aircon, washing machine or bathroom drain smell? Three smells, three different causes

A musty aircon, a sour washing machine and a sewer-like bathroom drain are three different problems. The aircon smells because a wet coil and damp dust grow mould, the washing machine because detergent residue and trapped water feed a film, and the floor drain because its water seal has dried out. Match the smell to the cause first, because each one has a different fix.

How can I tell which of the three is making the smell?

Go by what the smell is like and where you notice it. A musty or sour smell when the aircon starts points to the aircon, a sour or mouldy smell on clean laundry points to the washing machine, and a sewer or rotten-egg smell in a bathroom or kitchen points to a dry floor drain.

Johor Bahru makes all three more likely, because the air is humid for most of the year and anything that stays damp grows something. Our bathroom mould article covers that side of it.

Why does an aircon smell musty when it starts?

Because the cooling coil is wet every time the unit runs, and if it never dries properly, mould and bacteria grow on it and the first blast of air carries the smell into the room. Dust on a clogged filter makes it worse, because less air crosses the coil, so it stays wet longer.

Three places can be responsible, and they are not equally easy to reach:

1. The filter. Dust sits on it, and damp dust grows mould. This is the only part you can reach with no tools.
2. The evaporator coil and blower. This is behind the filter. SAYS Malaysia says dirt can harden there and mould can grow where a normal wash cannot reach.
3. The drain pan and drain line. Water collects here and, if it is blocked, sits and breeds mould. One US filter-maker's guide describes algae blocking the drain line on central systems. Wall-mounted split units in Malaysian homes have a drain hose rather than that setup, so we would leave this one to a technician.

What you can do yourself: pull the filter, rinse it under the tap, let it dry fully in the shade and put it back. Daikin Malaysia recommends every two weeks, and at least monthly if that is unrealistic. Never put a wet filter back. If the smell is still there after a clean, dry filter, the problem is the coil, and that is a servicing job.

What it costs: SAYS Malaysia quotes RM99 to RM120 per unit for a standard wash, RM150 to RM250 for a chemical wash and RM250 to RM400 for a chemical overhaul where the indoor unit is taken off the wall, with more for heavily blocked units. It suggests a standard wash every three to four months and a chemical wash about once a year for heavy users, or sooner if you notice a musty smell on startup, weak airflow or dripping.

One more habit: the filter-maker guide says mould struggles to grow when indoor humidity stays under about 50 to 60 percent, which is hard to reach in Johor Bahru. If your unit has a fan-only mode, use "auto" rather than leaving the fan running constantly, so the coil gets a chance to dry between cooling cycles.

Why does a front-loading washing machine smell?

Because the rubber door seal holds a fold of water, and detergent and fabric residue stick to it, so a damp film builds up that gives off a sour, mouldy smell. The drum itself is rarely the problem. The smell is in the places the water does not leave.

According to a practical guide to front loaders, the usual hiding places are:

• The bottom fold of the door seal, where water, lint, hair and small items collect.
• The detergent drawer and its recess, where residue builds up.
• The pump filter behind the small flap at the bottom, which catches lint and hair.
• The closed door. One manufacturer is quoted as saying a closed door causes odours, because the inside stays damp.

Cold, short washes with too little detergent leave more film behind, and wet laundry left sitting in the drum after the cycle adds to it.

The routine is short. After each wash, wipe the seal and leave the door ajar. Once a month, run an empty hot cycle with your brand's cleaning programme or a hot cotton wash. About every two months, clean the drawer and the pump filter (Electrolux suggests every two months, Samsung five or six times a year). Whirlpool says to wipe the seal after every cycle and deep clean it about monthly or every 30 cycles.

A caution: check your manual before using vinegar, because one maker says it damages rubber parts, and never mix cleaning products. Our guide to which products you must never mix explains why.

If the machine leaks, will not drain or shows an error, stop and call the maker's service line.

Why does a bathroom floor drain smell like a sewer?

Because a floor drain only works as an odour barrier while a small plug of water sits in its bend (the trap), and when the drain is not used, that water evaporates and drain gas comes straight up. There is no mould and nothing dirty to scrub. The barrier is simply gone.

A drain that is rarely used is the classic case: a guest bathroom, a spare room, a unit that has been empty for a few weeks. Rector Seal, a US plumbing-products maker, also notes that dry traps let in drain flies and sewer gnats, which can lay eggs in damp pipework. The sources we found do not give a number of days for how long it takes to dry out, since it depends on heat and airflow, so do not trust a fixed rule.

The fix is almost free: pour a jug of water into every floor drain, basin and shower drain in the home. Rector Seal suggests refilling "every few weeks" for rarely used drains. A monthly jug of water on any bathroom you use only occasionally is a simple habit.

If you refill the trap and the smell returns within a day, the water seal is not the problem. That is a leak or a venting fault, and it needs a plumber, not a cleaner. We covered the empty-unit version of this in why a vacant homestay smells.

Do I need to pay someone to fix these smells?

For two of the three, no. Rinsing an aircon filter, wiping a washing-machine seal and pouring water into a floor drain take about 15 minutes in total and cost nothing, and we would not suggest paying anyone to do them. Our frequency table has the schedule in one page.

The third is different. If the filter is clean and the aircon still smells, the coil needs a technician, and that is an aircon servicing job, not a cleaning one. We do not list aircon chemical washing as a service.

Where we can help is when these jobs have been skipped for a long time across the whole home, together with the grease, grout and tracks that go with them. That is what our deep clean covers, and it is quoted on request after we see the home. See our services for what is on offer, or read the 12 spots almost nobody cleans first and decide how much you want to do yourself.

Sources

• SAYS Malaysia, Is a chemical wash necessary for aircon servicing in Malaysia?
• Filterbuy, Why does my air conditioner smell musty and how to remove it
• Simply Maid, How to clean a washing machine
• Rector Seal, How to deal with infrequently used floor drains